Accurately Assessing Calcium Homeostasis in Dogs and Cats

Calcium exists in 3 forms in serum (ionized, protein-bound, and complexed with anions). Only ionized calcium (iCa) is metabolically active and therefore reflects the true calcium status. Four indicators of calcium status are available to veterinarians:

1. Total calcium (tCa) is widely available but does not always reflect the true calcium status of dogs and cats. Specific situations (e.g., hypoalbuminemia or hyperphosphatemia) make its interpretation very challenging. Regardless of these specific situations, tCa might under detect or over diagnose calcium disorders in many animals.
2. Formulas that adjust tCa to albuminemia or proteinemia have been developed but are unreliable. Currently, their use is not recommended in veterinary medicine.
3. Direct iCa measurement is considered the gold standard to determine the calcium status. Ionized calcium can be measured in house using calcium-sensitive electrode in benchtop blood gas analyzers or point-of-care analyzers or can be sent to external laboratories. Its measurement might be limited by technical, analytical and financial constraints. Pre-analytical factors especially must be taken into account for proper interpretation, otherwise clinically relevant errors, potentially harmful for the animals, might occur.
4. Finally, iCa can be estimated by a multivariate model (predicted ionized calcium) using routine biochemical and signalment variables. Although its use has been validated in dogs and cats, some rules must be followed to properly interpret the results. This lecture will help clinicians understand the limits of each indicator and properly interpret the results in the context of clinical settings through a clinical approach recommended by the author.